Another issue, with the cannon has been the slow fill time. Even when connected straight to the 150psi shop air line, it takes about 15 seconds for air to stop hissing through the check valve in the piston back (you can hear it). The check valve I made simply wasn’t moving enough air.
I made a couple of changes to improve the flow rate. First I drilled smaller hole out until it was only barely smaller than the ball. I also removed the spring, which wasn’t necessary.
At this point I realized the ball would definitely plug the hole in the retainer plate. To Fix this, I replaced the plate with an S shape piece of aluminum tig rod. If this works, I’ll make a nicer version at some point.
And.. it totally does work. I reassembled the cannon and tried filling it. Fill time is now about 2 seconds. the cannon even gets slightly warm from the compression.